How do I calculate the monthly payment......

Loan is $26,000.00
Down payment is 5,000.00
11% for 36 months

Pt = Po*r*t / (1 - (r + 1)^-t).

Pt = Principal after time t(36 mo.).
Po = Inital Loan amount.
r = (APR / 12) / 100 = (11/ 12 ) / 100 = 0.009167 = Monthly percentage rate
expressed as a decimal.
t = Length of loan in months.

Pt = (26000 * 0.009167 * 36) / (1 -(1.009167)^-36 = 8580.31 / 0.2800 = 30643.96.

Monthly payments = 30643.96 / 36 = 851.22